The present invention relates to circuits, and more particularly to voltage clamping devices that enable the use of low-voltage devices in high-voltage circuits.
In many applications, a high-voltage supply operates not only high-voltage devices, but also low-voltage devices as well. This situation is especially common in high-voltage integrated circuits. Most high-voltage integrated circuit processes offer both a wide variety of relatively compact low-voltage devices, and a smaller variety of larger high-voltage devices. The majority of a high-voltage integrated circuit consists of low-voltage circuitry. Therefore, some mechanism needs to be employed to protect the low-voltage components from excessive differential voltages. One common mechanism to protect a device against excessive voltage is a parallel-connected Zener diode (or a stack of such Zeners), popularly called a xe2x80x9cZener clampxe2x80x9d.
FIG. 1 illustrates a prior art high-voltage supply system 10 that employs a Zener clamp 16. A high voltage VHIGH (e.g., 200-400 volts) is provided to system 10 and is employed to directly power high-voltage devices 12 and to indirectly power low-voltage devices 14. A Zener clamp 16 is formed from Zener diodes D1 and D2, and is connected in parallel across the low-voltage devices 14. If the voltage across the low-voltage devices 14 exceeds the sum of the breakdown voltages of the Zener diodes D1 and D2, these diodes clamp the voltage across the low-voltage devices 14 to VLOW. Depending upon the desired value of VLOW, the Zener clamp 16 may contain more or fewer series-connected Zener diodes. The Zener clamp of FIG. 1 has limited application because it needs to be placed in series with some current-limiting element, such as a resistor R1. Many circuits require that the voltage across a device be clamped without restricting the current. FIG. 2 illustrates an alternate prior art high-voltage supply system 20 that achieves this end.
In FIG. 2, the high-voltage supply system 20 employs one or more low-voltage devices 24. A high voltage VHIGH (e.g., 200-400 volts) is provided to the system 20 and is employed to directly power high-voltage devices 22 and to indirectly power low-voltage devices 24. The low-voltage devices 24 are protected by a cascode MOSFET transistor M2. The gate of M2 is connected to a first Zener clamp 26 comprised of a Zener diode D5 and a Zener diode D6. Zener clamp 26 is biased by current flowing through resistor R2, and also through either resistor R3 or Zener diodes D3 and D4, in a manner which will be explained below. If the Zener diodes D5 and D6 have a breakdown voltage VZ, then the voltage seen across the low-voltage devices 24 equals:
VX=VINxe2x88x92VGS2 if VIN less than 2VZxe2x80x83xe2x80x83EQ. 1
VX=2VZxe2x88x92VGS2 if VINxe2x89xa72VZxe2x80x83xe2x80x83EQ. 2
A system employing a single cascode transistor suffices for voltages that do not exceed the breakdown voltage of the cascode transistor M2. For systems that do exceed this limit, a plurality of cascodes can be coupled in series.
In FIG. 2, a second cascode transistor M1 is coupled between the high input voltage VHIGH and transistor M2, such that the second cascode transistor M1 limits the differential voltage seen across the first cascode transistor M2. The gate of M1 is connected to a second Zener clamp 28 comprised of Zener diodes D3 and D4. Zener clamp 28 is biased by current flowing through resistor R2. Zener clamp 28 limits the voltage VDS2 seen across transistor M2 to
VDS2=VGS2; if VHIGH less than 2VZxe2x80x83xe2x80x83EQ. 3
VDS2=[(R3/(R2+R3)*(VHIGHxe2x88x922VZ)]xe2x88x92VGS1+VGS2;xe2x80x83xe2x80x83EQ. 4
if 2VZxe2x89xa6VHIGH less than 4VZ+2R2/R3VZ
VDS2=2VZxe2x88x92VGS1+VGS2; if VHIGHxe2x89xa74VZ+2R2/R3VZxe2x80x83xe2x80x83EQ. 5
Resistor R3 provides a path for current to flow around Zener diodes D3 and D4 to bias Zener diodes D5 and D6. At higher voltages, the current flowing through resistor R3 is augmented by additional current flowing through Zener diodes D3 and D4. The sum of both of these currents then flows through Zener diodes D5 and D6 to ground. Progressively higher voltages can be achieved by adding additional cascode stages. Furthermore, the number of Zener diodes in Zener clamps 26 and 28 can be decreased or increased to adjust to the needs of a specific application. Likewise, MOSFET transistors M1 and M2 can be replaced with bipolar junction transistors without significantly altering the operation of the circuit.
A stack of cascode stages such as those depicted in FIG. 2 can be termed a xe2x80x9ccompliance stackxe2x80x9d. Compliance refers to the ability of a system to adapt to externally imposed conditions. In this case, the circuit of FIG. 2 has an enhanced ability to adapt to externally applied voltages due to the presence of the cascodes, or in other words, the cascodes provide voltage compliance.
The Zener clamp of FIG. 1 does not provide voltage compliance, rather, it is the series current limiting element (R1 in this case) that provides voltage compliance. The circuit of FIG. 1 draws large amounts of current at higher voltages. The compliance stack of FIG. 2 is superior to the Zener clamp of FIG. 1 because the current drawn by the protected circuitry need not flow through the resistors, allowing very large resistances to be employed. Still, resistors R2 and R3 must draw some amount of current to overcome the effects of junction leakage, and the current they conduct increases at higher voltages. Since many modem integrated circuits are expected to operate on very small currents ( less than 10 xcexcA), it becomes difficult to simultaneously provide enough current at low voltages to bias the circuit, and yet to limit the current flow to acceptable levels at higher voltages. For this reason, systems of the sort illustrated in FIG. 2 are unsuited for low-current applications.
The following presents a simplified summary of the invention in order to provide a basic understanding of some its aspects. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention relates to systems and methods for limiting voltage to low-voltage devices (e.g., amplifiers, current limiters and logic) in high-voltage applications (e.g. optical switching, high-voltage drivers, dimmers, and video displays) where a high-voltage supply feeds low-voltage devices. The systems and method employ voltage limiting devices (e.g., Zener clamps) to bias one or more cascode devices (e.g., MOSFET transistors). The cascode devices are serially connected from a high-voltage supply to a low-voltage node. A primary cascode and a primary voltage limiter cooperate with a primary current source to assure that the voltage provided to the one or more low-voltage devices is within the applicable voltage limits. Additional cascodes and voltage limiters may be added to ensure that the maximum voltage rating of the primary cascode device is not exceeded.
In one aspect of the invention, the voltage limiters are series-connected Zener diodes that bias a string of series-connected cascode devices. A string of series-connected current source devices (e.g., depletion MOSFET""s) provide bias for the voltage limiters. Each of the current sources is set to conduct a current, IMIN, sufficient to bias a voltage limiter. The current drawn by the entire bias network, consisting of both the voltage limiters and their associated current sources, is substantially equal to the current drawn by one current source, or IMIN. Manufacturing variations, as well as the magnitude of the supply voltage, will determine the exact path taken by the bias current IMIN from the high-voltage supply to ground, such that the biasing of the voltage limiters is nondeterministic.
These aspects are indicative of but a few of the various ways in which the principles of the invention may be employed. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.